A self tapping bolt, by which a mounting member is fastened to a receiving member while internal threads are being cut in a tapping hole provided in the receiving member so that an independent step of cutting of internal threads can be obviated, is often used for fastening of automobile parts in order to save costs.
FIG. 6 is a sectional view showing fastening of a mounting member and a receiving member with a self tapping bolt. A self tapping bolt 200 reaches a mouth of a tap drill hole 202a provided in a receiving member 202 via a through hole 201a provided in a mounting member 201. If rotational torque is then applied to a head portion 200a, a cutting portion 200d cuts an inner circumferential surface of the tap drill hole 202a to form internal threads thereon and the self tapping bolt 200 is screwed into the tap drill hole 202a simultaneously. When a bearing surface of the self tapping bolt 200 comes into contact with the mounting member 201, joint force begins to act thereon, and then the mounting and receiving members 201, 202 are fastened by the self tapping bolt 200.
Self tapping bolts are generally categorized into two types in terms of method for forming of threads, a cutting type and a rolling type. The former employs cutting and the latter rolling for forming internal threads, respectively.
A self tapping bolt of rolling type, which employs a thread rolling die to exert plastic deformation on the surface of a tap drill hole in order to form internal threads, leaves residual stress therein. This residual stress may cause a problem when a mounting member of low creep strength (e.g. magnesium alloy) is fastened by the self tapping bolt. Creep caused by the residual stress stored during forming of the internal threads and compressive force acting on the mounting member will gradually degrade the joint force of bolt.
Since the material like this for the mounting member generally has a higher coefficient of thermal expansion than that of the bolt, reduction in joint force due to creep poses a problem for elements of engine which are exposed cyclically to high and room temperature conditions. The phenomenon will develop in the following steps. Under high temperature conditions the mounting member experiences thermal expansion which is restricted by the bearing surface of bolt. The bearing surface thus exerts compressive force on the mounting member, thereby developing creep therein. When the temperature of bolt and mounting member then returns to room temperature, the joint force applied to the mounting member by the bearing surface will get smaller than that before exposure to the high temperature environment as the mounting member experiences contraction leaving the creep.
On the other hand, as internal threads are formed by cutting in a tap drill hole provided in a receiving member, a self tapping bolt of cutting type leaves less residual stress in the formed threads. The creep generated in the internal threads can in this way be reduced under the high temperature conditions, and thereby a reduction in joint force (bolt loosening), which occurs when the temperature returns to room temperature, can be restrained better compared with a self tapping bolt of rolling type.
A self tapping bolt according to the prior art has posed a problem that internal threads are unavoidably cut again when a fastened bolt needs to be unscrewed. A typical self tapping bolt of cutting type according to the prior art is shown in FIG. 7.
A self tapping bolt 100 shown in FIGS. 7A and 7B has a cutout 101, the cross section of which occupies a quarter of a circular cross section of threaded portion. When the self tapping bolt 100 is screwed into a tap drill hole in a rotational direction shown by the arrows in the figures, an end surface 101a of cutout 101 cuts a surface of tap drill hole provided in a receiving member, forming internal threads. If the occasion arises that the self tapping bolt 100 needs to be unscrewed, the other end surface 101b unnecessarily cuts the internal threads having already been formed.
A self tapping bolt 110 shown in FIGS. 7C and 7D has three cutouts 111, which are spaced at circumferentially equal intervals. When the self tapping bolt 110 is unscrewed, an end surface 111b of each of cutouts 111 also cuts internal threads having already been formed, like the self tapping bolt 100.
In this way, self tapping bolts such as 100 and 110 have a problem that they can not withstand repeated handling, since the internal threads get thinner each time unscrewing of bolt is conducted, resulting in degradation of dimensional accuracy of threads and a gradual reduction in joint force.